In stereoscopic, or three-dimensional, photography, it is necessary to record simultaneously, images of the original scene taken from two different points of observation. As is well known, the sensation or perception of depth in a person's field of vision results, in part, from the viewer's interpretation of the size of viewed objects and, in part, from the reception of two separate images, one each by the left eye and by the right eye. Because the eyes are spaced apart, each eye views the original image from a different vantage point. The received images are thus different, and these are processed and combined by the brain to provide depth and distance information.
To faithfully reproduce the stereoscopic nature of the original scene by photography requires that separate left-eye and right-eye images be recorded on the film. In early attempts to recreate three dimensional effects by motion picture photography, these separate left-eye and right-eye images were recorded on separate films transported through separate motion picture cameras that were physically spaced apart. The original scenes were then reproduced by running the two films simultaneously through separate motion picture projectors operating in synchronism and casting upon the screen for each frame of film these two images in optical registration. The images projected onto the screen were polarized, so that a viewer wearing polarized glasses would receive the correct image at each eye.
The early results of three-dimensional motion picture photography, using separate cameras to record the original scene on separate films, and then using separate motion picture projectors to recreate the image in the theater, were only marginally successful from a technical point of view. Although the three-dimensional effects were sometimes astounding, watching early stereoscopic films was often uncomfortable to the viewer and gave rise to eye strain. The images were not sharp and clear, for it was operationally impossible to maintain proper registration and synchronism in recreating the orignal scene by projection.
More specifically, perfect reproduction required not only that the two films be running at precisely the same speed, but also that each frame pair be passing through the projection gate at the same instant of time and that the projected images be in focus and aligned both vertically and horizontally. It is not surprising that all of those conditions were rarely, if ever, met. As already noted, the end result was pronounced discomfort of the viewer due to the changing and erratic nature of the perceived left and right images.
In an effort to overcome the aforementioned difficulty of obtaining synchronized and registered images from separately projected films, more recent stereoscopic motion picture cameras have utilized a different film format. In the newer systems, the left and right images are recorded on a single film in different frames or frame areas. One such format that is generally preferred over others is one in which the corresponding left and right images of each action frame are recorded in adjacent longitudinal halves (i.e., normally the vertical halves) of a bisected 35 mm. film frame. This means that in the customary 35 mm. format in which a frame extends for a longitudinal distance corresponding to four perforations, each half-frame will be two perforations in length or pitch. With the left and right ocular images recorded in this manner, the half-frames have a narrow longitudinal dimension and a wide lateral dimension. This format produces half-frames having an aspect ratio of 2.35:1, which is advantageous because that aspect ratio corresponds to the one customarily used for wide-screen Cinemascope.
The optical systems for recording scenes in the above format consist of a pair of receiving lenses that are spaced apart by a distance corresponding to some standard or average spacing between the left and right eyes of the viewer. Thus, the left receiving lens receives images that would normally be seen by the left eye, and the right receiving lens receives images that would normally be seen by the right eye of an observer. Both receiving lenses in the prior art optical apparatus of which we are aware arranged on a common horizontal plane (as are an observer's eyes). At the interior of the optical apparatus, the left and right images are redirected by optical means to the motion picture camera or camera lens to form the photographic images on the film, as described above.
Where, as in the case of the above-described photographic apparatus, the receiving lenses are spaced apart along a common horizontal plane, it is necessary to rearrange these images so that they fall on adjacent longitudinally displaced areas to the film. This requirement is in addition to that of redirecting the left and right images so that they are laterally centered on the film. The longitudinal displacement of the two ocular images is usually done by prisms. The disadvantage in doing this is that the image must pass through several optical interfaces (i.e., the interface between the optical element and air or adjacent optical element). This results in a loss of image intensity and a loss in image sharpness. Image intensity is lost with each passage through an optical interface because a small portion of light is reflected at each interface and therefore not transmitted to the next segment or element of the optical system. Image sharpness is lost in prisms due to unavoidable diffraction in passing through two or more pristatic elements.